Phase I: SPECT is non-invasive imaging technique which provides an image of the three dimensional distribution of radionuclide in any slice through an organ under study. These images allow investigation of metabolism in the region being studied and are a useful tool for diagnosing symptoms of diseases such as cancer and myocardial ischemia. SPECT is also showing promise in imaging of biological processes in small laboratory animal in vivo. Recently, there have been significant advances in development of human disease models in such small animals. High resolution SPECT studies of small animals can therefore improve our understanding of human diseases. At present, the detectors used in SPECT systems often limit their performance (for clinical and small animal imaging). Modern SPECT detectors consist of scintillators coupled to PMTs. Important requirements for scintillators used in SPECT include high light output, high energy resolution, decent stopping efficiency, fast response, and low cost. At present, no established scintillator meets all these requirements. The goal of the proposed effort is to investigate the feasibility of using a new scintillator in SPECT imaging in the Phase I research and build and evaluate a small animal SPECT system using arrays of such a scintillator in the Phase II project. Nuclear medicine including single photon emission computed tomography and positron emission tomography, radiology, non-destructive testing, materials research, X-ray diffraction, nuclear and high energy physics research, astronomy, and geological exploration. Phase II: Nuclear medicine techniques (such as SPECT and PET) are becoming powerful new tools in imaging biological processes in small laboratory animals. With the ever increasing number of human disease models including cancer, particularly in the smaller animals such as mice and rats, the potential of high resolution nuclear medicine technology to contribute unique information is becoming apparent to many researchers. The critical advantage of nuclear medicine procedures is that they allow functional information to be obtained non-invasively, so each animal can be studied repeatedly. Thus, it is clear that nuclear medicine imaging of small animals is highly desirable. Clinical radionuclide systems are not suitable for imaging small animals because their spatial resolution is inadequate. Hence, dedicated, low cost instruments are required for conducting small animal studies with higher spatial resolution than what is currently achieved with clinical SPECT scanners. The goal of the proposed effort is to build and evaluate a high resolution small animal SPECT system using arrays of a promising new scintillator coupled to silicon avalanche photodiodes. Nuclear medicine including single photon emission computed tomography and positron emission tomography, radiology, non-destructive testing, materials research, X-ray diffraction, nuclear and high energy physics research, astronomy, and geological exploration.